High-frequency drying



HIGH-FREQUENCY" DRYING Filed Nov. 2, 1944 .V n im i c. J. MERRMM2,473,539

Patented June 21, 1949 HIGH-FREQUENCY DRYING Charles J. Merriam,Winnetka, Ill., assgnor, by mesne assignments, to Guardite Corporation,Wilmington, Del., a corporation of Delaware Application November 2,1944, Serial No. 561,626 8 Claims. (Cl. 34-1) This invention relates tohigh frequency drying of a product which has been wet by a volatilefluid.

One of the disadvantages of ordinary drying methods is that all heatinput is from the outside of the product to the inside. As a result, theouter portions of the product dry first and are at the same timesubjected to the higher temperature. Thus there is always a gradient inmoisture content from the inside out, and in addition it is dimcult toprevent scorching of the outer portions of a product because it ishotter than the inside.

This is particularly true in spray drying operations in which an articleis dried from a flowable wet condition to a ilne discrete solid form ina single spraying operation during which all of the particles areunsupported except by the atmosphere in which they are operating. Inordinary spray drying operations of this type it is not practicable todo anything except to contact the falling particles with acountercurrent now of hot gases and or with radiant heat. Thisaccentuates the problem of preventing overdrying of the outer portionsof each particle.

Two forms of apparatus for carrying out the process are showndiagrammatically in Figs. 1 and 2; Fig. 1 being a vertical sectionthrough a spray drying chamber and Fig. 2 being a vertical sectionthrough a different form of spray drying chamber.

In the present process' such a spray drying operation is carried out bysupplying substantially the entire heat input at the critical stage cidrying by means of a high frequency current generated within theparticles so that the heat gradient is from the inside out. At the sametime the operation is carried out in an atmosphere Within which thetemperature is regulated, preferably to produce, at least at thecritical times, a temperature below that of the outside of the particlesbeing dried.

Several results may be accomplished by this regulation. In the hrstplace it is possible to overdry the central portions of each particlebeyond the finally desired moisture content. The excess amount ofmoisture removed from the interior of the particles may be considered ascondensed on the cooler outer layers which are then cverwet. After thespray drying operation the 2 particles may be normalized by merestanding so that the moisture may be equally distributed therethrough.

This particular method of procedure has great value Where it is notpossible, or desirable, to remove all air from the drying atmosphere. Itproduces the wettest and coolest part of the product'where the exposureto the air occurs. The relatively hot interiors of the particles are notin contact with oxygen and, therefore, are not subject to many forms ofdeterioration which could occur on the outside of the particles.

Furthermore in a spray drying operation in which a cool gas is used toregulate the temperature of the outside of the particles, the gas mayreadily be introduced in countercurrent flow to the fall of the spray insuch manner that it acts to cool the particles in the latter portion oftheir fall, when they require cooling, but serves as a heating anddrying medium in the earlier4 or upper regions of the spray.

Furthermore, if the cooling gas is steam, it may in the latter stages beused to regulate evaporation, and, being cooler than the particles, willactually receive moisture therefrom even though the steam is saturatedat the time it is employed. In such case, however, where saturated steamis used in this manner, it may not be desirable to use a countercurrentflow as such aflow will tend to produce condensation on the coolerparticles in the initial phases of the spray because of the fact thatthe steam will be heated up by heat evolved from the particlesthemselves.

The invention is preferably carried out in an atmosphere substantiallyfree from air and at low atmospheric pressure, as, for example, in theneighborhood of 2 to 4 inches of mercury absolute.

The process may be employed in combination with a drying current of hotgas. For example, the product may be introduced in discrete particlesfrom an atomizer concurrently directed with a blast of superheated steamor other gas.

An apparatus for carrying out this example of' the process isdiagrammaticlly illustrated in Fig 2, in which I0 is a spray dryingchamber having an inlet li and an outlet l2. Hot gas is introducedthrough the inlet Il and flows in the manner indicated by the arrowsdownwardly through the chamber and then outwardly through the outlet I2.Particles to be dried are introduced to the chamber through a nozzle I3and gravitate through the chamber to the bottom where they may beallowed to accumulate for future removal. A plurality of electrodes I4are provided about the chamber intermediate the top and bottom thereof,and are connected by suitable means to an electromagnetic ileld creatingdevice (not shown). The amount of the steam, its temperature, and itsrate of flow, are correlated to the amount of product, the moisturecontent thereof, and to the amount of current supplied so that duringthe process the temper- -ature of the exterior of the product passesthat of the steam. Such a process has the advantage of using the steamas a heating and drying medium during the first part of the process andas a cooling medium in subsequent parts. It is obvious that the highfrequency current does not have to be employed during the entire cycle.It may be omitted during the early stages thereof and supplied in alaterv portion of the cycle. The cycle may be considered as completewhen the supply of high frequency current ceases, even though the steammay remain in contact with the particles vfor some time thereafter.

' Such a process also has the advantage that the product is dried morerapidly on the exterior at first, so that the moisture content of theinterior initially rises with respect to the exterior of each particle.The rate of drying then changes as the superheat of the steam is lost,and ultimately the exterior of the product becomes markedly wetter thanthe interior. It will be noticed that this reversal of the relativemoisture content of the interior and exterior may be accomplished eventhough the concurrency of the steam flow is not continued to the end ofthe process. For example, spray may be introduced horizontally in acurrent of steam, and, as the particles pick up a vertical component ofow, the steam may be withdrawn to the side, and the particles permittedto fall through a quiescent atmosphere or may .even be carried in a sidecurrent of gas.

An apparatus for carryingvout this example of the process is illustrateddiagrammatically in Fig. 1, in which IIO is a flow chamber having aninlet I II and an outlet IIZ. Hot gases are introduced to the chamberthrough the inlet and flow more or less horizontally across the chamberand out the outlet II2. Particles to be dried are introduced -to theflow through the nozzle IIS. The particles after leaving the nozzlegravitate downward through the chamber and are accumulated in thebottom. A plurality of electrodes II4 are provided which are connectedby suitable means to an electromagnetic ileld creating device.

When the concurrency of the steam or gas flow is carried out throughoutthe spray drying process. as particularly where the ow of the materialis substantially vertically downward, a very desirable heat gradient isestablished from the interior of the product to the steam. That is, theinterior of the product is hottest, the temperature of the exterior ofthe particles is intermediate, and the steam is cooler than the exteriorof the particles.

The foregoingl detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom.

What I claim as new, and desire to secure by Letters Patent, is:

1. The method of drying moist particles of a substantially.non-volatilematerial which comprises passing the material in unsupported concurrentflow by gravity in a gas substantially hotter than the particles,wherein the surface of the particles is dried, and supplying, at leastduring a latter portion of the flow in said gas, high frequency currentto the particles, whereby they are internally heated to a temperaturehigher than that of the exteriors thereof.

2. The method of drying moist particles of a substantially non-volatilematerial which comprises passing the material in unsupported concurrentlow by gravity in a gas substantially hotter than the particles, whereinthe surface of the particles is dried. and supplying, at least during alatter portion of the flow in said gas, high frequency current to theparticles, whereby they are internally heated t'o a temperature abovethe then temperature of the gas.

3. The method of drying moist particles of a substantially non-volatilematerial which comprises passing the material in unsupported concurrentflow by gravity in a gas substantially hotter than the particles,wherein the surface of the particles is dried, supplying, at leastduring a latter portion of the flow, high frequency current to theparticles, whereby they are heated, and correlating the rate of flow,temperature, and quantity of gas, the amount and moisture content of theparticles, and the amount of high frequency current supplied thereto, sothat the gas at a later stage of the flow has a lower temperature thanthe surface of the particles of the product to condense moisture on saidsurface.

fi. The method as set forth in claim 3, in which the moisture is water.

5. The method of spray drying a water-wet product which comprisesatomizing the product in a current of steam hotter than the particles,and heating the particles while suspended in steam by a high frequencyfield until they are substantially hotter than the steam.

6. I'he method of drying a product containing a volatile liquid whichcomprises producing a high-frequency iield within the product to bedried whereby it is heated and the heat so produced Volatilizes water inthe product, characterized by supplying steam to the product duringdrying, the temperature of said steam being lower than thetemperature ofsaid product at the outer surface thereof, whereby the exterior of theproduct is maintained at a temperature markedly below the interiorthereof and at a moisture content above the interior thereof.

7. The method of drying water-moist particles of a substantiallynon-volatile material which comprises passing the material in concurrentflow in steam which is substantially hotter than the particles, whereinthe surface of the particles is dried, and supplying, at least during alatter portion of the flow in said steam, high frequency current to theparticles, whereby they are internally heated to a temperature above thethen temperature of the steam.

8. The method of drying water-moist particles of a substantiallynon-Volatile material which comprises passing the material in concurrentflow in steam which is substantially hotter than the particles, whereinthe surface of the particles is dried, and supplying, at least during alatter portion of the flow, high frequency current to the particleswhereby they are heated, and correlat- 5 in; the rate of now,temperstme. and quantity oi' steun. the amounts and water content of theparticles. `9,1111 the amount oi' high frequency current suppliedthereto, 5o that the steam at a later stale oi'y the now has 9. lowertemperature than the surface oi the particles o! the product to con- 5dense water on said surface.

CHARLES J. MERRIAM.

REFERENCES drum The tolluwinc references are of record in the me of thispotent: l

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